Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant, high speed. Data are stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs.
A voice coil motor (VCM) is used to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from an actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The VCM includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When current is applied to the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move relative to the permanent magnets, causing the actuator body to pivot about the pivot shaft and move the heads across the disc surfaces.
Typically, the plurality of open-centered discs and a plurality of spacer rings are alternately stacked on the hub of a spindle motor. The hub, defining the core of the stack, serves to align the discs and spacer rings around the common centerline. Movement of the discs and spacer rings is typically constrained by placing the stack under a compressive load and maintaining the load by means of a clamp ring. Collectively, the discs, spacer rings, clamp ring and spindle motor hub define a disc pack assembly or disc pack.
The discs must be clamped to the hub arrangement with sufficient force to reliably prevent radial movement of the disc pack, which could result from unbalanced torsional forces, thermal expansion or sudden load changes during transportation. Moreover, when the discs are not aligned perfectly concentric with the hub to provide a balanced disc pack assembly, the discs can vibrate during rotation and cause errors in track following as well as create excessive noise. In particular, increasing demands for high-performance, large capacity disc drives have lead to increased disc rotational speeds. At such high rotational speeds, improper balancing of the disc pack about the spindle motor can cause excessive bearing wear, eccentricity resulting in the reading or writing on adjacent concentric tracks on the disc surfaces, and surface runout that may result in "head crashes" and the consequent damage of both the heads and the magnetic medium on the disc surfaces. Unbalance can also set up undesirable vibrations which can be transmitted to system cabinets, creating annoying acoustic noise as well as providing undesirable vibrational inputs to adjacently mounted disc drives.
The condition of unbalance of a disc pack assembly can be classified as static and dynamic unbalance (also sometimes referred to as static imbalance and dynamic, or coupled, imbalance). In the case of static unbalance, the unbalance force appears in a single axis plane and in the same direction as the unbalance mass. Such unbalance can cause translational displacement of the discs during rotation as the normal circular rotation of the discs becomes exaggerated. As will be recognized, the disc pack assembly is at least in principle, cylindrically symmetrical with respect to the rotation axis of the assembly; i.e., the axis of the hub. Consequently, in principle, the center of mass of the assembly will be located on the rotation axis and the assembly will be statically balanced for rotation on the disc drive. However, factors such as small variations in disc thickness, discs that are not perfectly circular, central mounting apertures that are not perfectly concentric with the outer edges of the discs, and central mounting aperatures not concentric with the axis of rotation, can all contribute to static unbalance of the disc.
In the case of dynamic unbalance, the unbalance forces can be in a single axial plane and on opposite sides of the rotational axis, or in two different axial planes. During rotation, the two unbalance forces form a couple, which has a tendency to rock or tilt the axis of rotation, causing the dynamically unbalanced discs to slightly tilt as they rotate. Dynamic balance is achieved when the axis about which the discs are forced to rotate is parallel with a principle axis of inertia so that no products of inertia about the center of gravity of the body exist in relation to the selected rotational axis.
Prior art attempts to provide solutions for balancing disc pack assemblies, however, do not provide adequate remedies. For example, one approach involves placing pieces of adhesive backed lead foil in strategic locations on the assembly. One problem with this approach is that aging of the adhesive after extended operation of the disc drive could cause the foil to detach from the assembly. Moreover, the adhesive forms a source of large organic molecules which can accumulate on the heads and thereby impede head performance.
Another prior art approach involves drilling a series of symmetrically spaced holes in one end of the hub, tapping the holes, and setting screws of various lengths mounted in the holes to effect the final balance. However, such a method results in increased time and cost in manufacturing and installing the holes and screws. Moreover, the tapped holes can trap particulate matter that can later be dislodged and adhere to disc surfaces as surface flaws which impede the flight of heads over such surfaces.
In a third prior art approach, a plurality of discs are fixed to a spindle motor in a manner such that they are alternately shifted about the spindle motor so that their respective inner circumferential edges abut against the outer circumferential surface of the spindle motor hub at different angular locations. Generally, for an even number of discs, half of the discs can be shifted in one direction and the remaining number of discs can be shifted in the diametrical direction (i.e., 180 degrees out). For an odd number of discs, the discs can be angularly offset around the spindle motor hub at evenly spaced intervals (such as every 72 degrees for a disc pack having a total of five discs).
However, while such methodologies can generally achieve nominal static and dynamic balance for disc packs having four or more discs, a disc pack having only three discs presents particular difficulties. For example, angularly offsetting the three discs 120 degrees apart will often nominally achieve static balance, but not dynamic balance.
A continuing trend in the industry is to provide ever greater data recording densities and data transfer rates in successive generations of drives. As discs are among the highest cost components in a disc drive, there is a significant economic benefit to a disc drive manufacturer to be able to achieve a given data storage capacity with a reduced number of discs.
Thus, notwithstanding the balancing problems associated with three-disc configurations, manufacturers are increasingly developing disc drive products utilizing three discs. There is a need, therefore, for an effective and efficient approach to achieving both static and dynamic balancing of a three-disc disc pack.